The Minor Intrusions of Assynt, NW Scotland: Early Development of Magmatism Along the Caledonian Front
Total Page：16
File Type：pdf, Size：1020Kb
Mineralogical Magazine, August 2004, Vol. 68(4), pp. 541–559 The minor intrusions of Assynt, NW Scotland: early development of magmatism along the Caledonian Front 1, 2,3 4 K. M. GOODENOUGH *, B. N. YOUNG AND I. PARSONS 1 British Geological Survey, West Mains Road, Edinburgh EH9 3LA, UK 2 Department of Geology and Mineralogy, University of Aberdeen, Marischal College, Broad Street, Aberdeen AB24 3UE, UK 3 Baker Hughes Inteq, Barclayhill Place, Portlethen, Aberdeen AB12 4PF, UK 4 Grant Institute of Earth Science, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK ABSTRACT The Assynt Culmination of the Moine Thrust Belt, in the northwest Scottish Highlands, contains a variety of Caledonian alkaline and calc-alkaline intrusions that are mostly of Silurian age. These include a significant but little-studied suite of dykes and sills, the Northwest Highlands Minor Intrusion Suite. We describe the structural relationships of these minor intrusions and suggest a classification into seven swarms. The majority of the minor intrusions can be shown to pre-date movement in the Moine Thrust Belt, but some appear to have been intruded duringthe period of thrusting.A complex history of magmatism is thus recorded within this part of the Moine Thrust Belt. New geochemical data provide evidence of a subduction-related component in the mantle source of the minor intrusions. KEYWORDS: Assynt, Caledonian, minor intrusion, Moine Thrust, Scotland. Introduction north of Assynt, to the Achall valley near Ullapool, but they are most abundant in the Assynt area. The WITHIN the Assynt Culmination of the Moine minor intrusions constitute a significant part of the Thrust Belt of NW Scotland (Fig. 1) lie a variety of total volume of igneous rock in the Assynt area, Caledonian alkaline and calc-alkaline intrusions but have been little studied. The first detailed study that have been of interest to geologists since the was that of Sabine (1953) who divided the minor end of the 19th century (e.g. Bonney, 1883; Heddle, intrusions into six classes and studied their 1881; Horne and Teall, 1892; Teall, 1900; Teall, relationships to the regional structure. Subsequent 1907; Shand, 1910; Phemister, 1926). The main publications on the region (e.g. Parsons, 1979) syenite intrusions of Assynt, the Loch Borralan and have generally accepted the detailed work of Loch Ailsh plutons, have been well studied (e.g. Sabine; a review of the existingliterature is Parsons, 1965; Woolley, 1970; van Breemen et al., provided by Parsons (1999). The only recent 1979; Thompson and Fowler, 1986, Halliday et al., investigation of these intrusions was in a hitherto 1987; Thirlwall and Burnard, 1990; Parsons, unpublished thesis (Young, 1989). We present new 1999). Associated with them is a less well field, petrographical and geochemical data, on the understood suite of minor intrusions, formally basis of which we have revised the original known as the Northwest Highlands Minor classification scheme for the minor intrusions. Intrusion Suite, which includes sills and dykes The relationships between the different swarms that range in composition from lamprophyres to and their possible sources are discussed below. peralkaline rhyolites and nepheline-syenites. The intrusions of this suite extend from Loch More, Regional geology * E-mail: [email protected] The Assynt Culmination is the largest and most DOI: 10.1180/0026461046840207 geologically spectacular window through the # 2004 The Mineralogical Society K. M. GOODENOUGH ETAL. FIG. 1. Simplified map of the northwest of Scotland, showing the major geological structures. Moine Thrust Belt (Fig. 2) (Johnson and Parsons, have been disputed (Peach et al., 1907; Bailey, 1979; Elliott and Johnson, 1980; Coward, 1983, 1935; Sabine, 1953; Krabbendam and Leslie, 1985). West of the Assynt Culmination lies the 2004). Movingwest from the Moine Thrust, these undisturbed Foreland (Park et al., 2002), where are: the Ben More Thrust, with a sheet of the clastic sedimentary rocks of the Proterozoic Lewisian gneisses, Torridonian sandstones and Torridonian Supergroup overlie Archaean Cambrian quartzites in its hangingwall; the Lewisian gneisses along an irregular unconfor- Glencoul Thrust, carryingLewisian gneissesand mity. The Torridonian rocks are in turn overlain Cambrian quartzites; and the Sole Thrust, by a Cambro-Ordovician sequence, consistingof carryingduplexes of the Cambro-Ordovician thick quartzite units that pass up into fissile succession. The amount of movement on the mudstones (the Fucoid Beds) and gritty sand- Sole Thrust has been a cause of some debate stones, with the dolomitic limestones of the (Coward, 1985; Halliday et al., 1987) and this has Durness Group at the top of the sequence. To not yet been resolved. the east, the Assynt Culmination is bounded by The major igneous intrusions in the Assynt the Moine Thrust, east of which lie Proterozoic region are the alkaline Loch Borralan and Loch metasedimentary rocks of the Moine Supergroup. Ailsh syenite plutons. The Loch Borralan Pluton Within the Assynt Culmination, it is generally is in part silica undersaturated and highly potassic agreed that three major thrusts can be recognized (Woolley, 1970) whereas all units at Loch Ailsh (Fig. 2), although the exact lines of the thrusts are silica saturated and relatively sodic (Parsons, 542 EARLY MAGMATISM ALONG THE CALEDONIAN FRONT, SCOTLAND FIG. 2. Sketch map of the Assynt Culmination, showingthe approximate distribution of the main groupsof intrusive rocks. Thrust patterns from Peach et al. (1907), modified after Coward (1983) and Krabbendam and Leslie (2004). MT À Moine Thrust; BMT À Ben More Thrust; GT À Glencoul Thrust; ST À Sole Thrust. Locations of minor intrusions from Sabine (1953), modified after Young(1989) and recent BGS mapping.Note that this is not a comprehensive map of all the minor intrusion localities. 1965). Both plutons are Silurian in age: the Loch al., 1987). The Loch Borralan Pluton is Borralan Pluton has a U-Pb date of 430Ô4 Ma considered to post-date movements on thrusts (van Breemen et al., 1979); and the Loch Ailsh within the Moine Thrust Belt (Parsons and Pluton has been dated at 439Ô4 Ma (Halliday et McKirdy, 1983), whereas the Loch Ailsh Pluton 543 K. M. GOODENOUGH ETAL. TABLE 1. Summary of the modern rock names for the Assynt minor intrusions. Name given by Modern rock name Modern swarm Structural Sabine (1953) name relationship Canisp Porphyry Porphyritic quartz- Canisp Porphyry Swarm Pre-thrustingin microsyenite Moine Thrust Belt Ledmorite Melanite nepheline- Ledmorite Swarm Uncertain microsyenite Grorudite Peralkaline rhyolite Peralkaline Rhyolite Swarm Pre-thrustingin Moine Thrust Belt Hornblende- Hornblende microdiorite Hornblende Microdiorite Swarm Pre-thrustingin porphyrite Moine Thrust Belt Nordmarkite Quartz-microsyenite Nordmarkite Swarm Predates ductile movement on Moine Thrust Vogesite Vogesite Vogesite Swarm Pre-thrusting in Moine Thrust Belt Not recognized Porphyritic trachyte Porphyritic Trachyte Swarm Syn- to post-thrusting in Moine Thrust Belt has generally been considered to pre-date area for the Canisp Porphyry is on Beinn Garbh, to thrusting(Halliday et al., 1987). the south of Loch Assynt, where it forms a series of sills of which at least one is nearly 50 m thick. Structural relationships of the Assynt minor intrusions Vogesites Sabine (1953) divided the minor intrusions of The vogesites are green-grey-weathering Assynt into six main classes: ‘grorudite’; ‘Canisp medium- to coarse-grained hornblende-phyric Porphyry’; ‘hornblende porphyrite’; ‘nordmarkite lamprophyres that form sills up to many tens of porphyry’; ‘vogesite’ and ‘ledmorite’. We have metres thick. They occur throughout the Assynt updated this classification on the basis of new Culmination, but are most common in the field, petrographical and geochemical data (see Cambro-Ordovician dolomitic limestones of the Table 1 and also Parsons, 1999). New rock names Sole Thrust Sheet (Fig. 2). They are not seen are based on the BGS Rock Classification Scheme within the Foreland or east of the Moine Thrust. Volume 1: Igneous Rocks (Gillespie and Styles, The best evidence that these intrusions pre-date 1999), which is closely based on the IUGS thrustingcan be seen at the roadside on the north scheme (Le Maitre et al., 1989) and is now used shore of Loch Assynt (NC 240239), where an in all BGS maps and publications to ensure offshoot from a vogesite sill in dolomitic lime- consistency. Detailed field mappinghas givenus a stone is clearly imbricated. Some vogesite sills in clear understandingof the relationship of the the Loch Ailsh area have been hornfelsed (Young, different swarms to each other, and to the regional 1989) by the intrusion of the Loch Ailsh Pluton structures. (439 Ma; Halliday et al., 1987), which they must therefore pre-date. The vogesites are thus considered to be amongthe earliest intrusions in Canisp Porphyry the Moine Thrust Zone in Assynt. The Canisp Porphyry is a distinctive red feldspar- phyric quartz-microsyenite (Sabine, 1953). It has Hornblende microdiorites been recognized only in the Foreland (Fig. 2), close to the Sole Thrust but never east of it, and is Sabine’s ‘hornblende porphyrites’ have been generally considered to pre-date movement in the renamed hornblende microdiorites (Parsons, Moine Thrust Belt (e.g. Parsons, 1999). The type 1999). These are medium-grained rocks, which 544 EARLY MAGMATISM ALONG THE CALEDONIAN FRONT, SCOTLAND can be distinguished from the vogesites in hand of porphyritic trachyte cut across